1. Field of the Invention
The present invention relates to an easy opening can end used for a can end of a beverage can and a food can and more particularly to an easy opening can end made of steel sheet coated with resin layers, and to a method for fabricating the same.
2. Description of Related Art
An easy opening can end is widely used to open a can by breaking an opening section formed at a can end through pulling up a tab attached to the can end by use of fingers and the like. The above opening section is formed at the can end of the can accommodating various beverages of beer, juice, coffee and the like or food. The easy opening can end is largely classified as a partial opening can end mainly used for beverage cans and as a full opening can end mainly used for food cans. For the partial opening can end, there are a pull-top tab can end and a stay-on tab can end.
FIG. 1 is a plan view showing an example of the pull-top tab can end.
A tab 3 is secured to the center of a central panel section 11 of the can end 1 made of metal sheet of steel or aluminum alloy or the like by way of a rivet mechanism 9. By pulling up the tab 3 by fingers or the like, a working end of the tab 3 pushes down through an action of a lever a broken opening section 5, where a score 2 is engraved in the central panel section 11, so that the score 2 is broken. Furthermore, by pulling up the tab 3, a broken opening section piece may be completely separated to open the can end 1.
FIG. 2 is a plan view showing an example of the stay-on tab can end.
As similar to FIG. 1, when the working end of the tab 3 is pulled up subsequent to breakage of the score 2, breakage will progress and a part of the broken opening section piece is pushed down inside the can body as is connected to the can end 1, thus enabling the can end 1 to be opened.
On the other hand, in the case of the full opening can end, the score engraved around the outer circumference of the can end is broken by a pull up with fingers or the like of the tab attached to the panel section in the neighborhood of the outer circumference of the can end, and similar to the can end of the pull-top tab can end, the opening section piece is removed from the can end and the can end is opened.
Conventionally, as shown in FIG. 3, the score of such easy opening can end is formed by press forming to create a V-shaped cross-section by providing such high load to make the depth of the score having more than half thickness of the can end 1 by use of a tool 12 having a knife edge protrusion formed according to a contour of a predetermined opening section. At this time, in case steel sheet coated with resin layers is used, there is a problem that resin layers of the outside of the can end are subjected to damage and corrosion resistance is deteriorated. To avoid this, a counter measure is normally taken to prevent deterioration of corrosion resistance by providing supplementary coating after press forming. Of late, there is an example that a can end made of aluminum alloy is used, but it is high in cost and presents a problem of recycling.
In addition, a number of methods are presented to prevent resin layer damage caused when using steel sheet coated with resin layers. For instance, according to Japanese Laid-open Patent Applications Nos. 6-115546, 6-115547 and 6-115548, the methods for forming the score by compound extrusion are proposed. It is reported that by these methods resin layer damages do not occur and supplementary coating is not required. However, working condition of compound extrusion and the detail of forming the score are not clear and it is doubtful whether the score is stably formed. According to Japanese Laid-open Patent Application No. 8-99140, a method for forming the score is disclosed that by using upper and lower dies having the shoulder radius of 0.1 to 1.0 mm, the score is formed to have the thickness of its thinnest section to be not exceeding half of the original thickness. However, can opening force is determined by the absolute value of the thickness of the thinnest section of the score as well as by strength, so that it does not necessarily follow that good can openability may be obtained even if the thickness of the thinnest section is arranged to be less than half of the original thickness.
On the other hand, with respect to improvement of can openability for the easy opening can end made of steel sheet, for instance, Japanese Utility Model Registration No. 63-40439 discloses a method for broadening a clearance made between the central panel section of the can end and the finger picking section of the tab by forming a recessed part for inserting fingers to the central panel section below the finger picking section of the tab in order to facilitate insertion of fingers and picking of finger picking section. And also, Laid-open Japanese Utility Model Registration No. 5-40133 discloses a method that the tab is riveted in the degree of rotatably moving to the openable position where the central axis of the tab coincides with that of the broken opening section from the unopenable position where the central axis of the tab deviates from that of the broken opening section, and during the tab moves from the unopenable position to the openable position, the finger picking section of the tab is raised up by tapered protrusion set up at the central panel section located between the rivet and the finger picking section, thereby facilitating insertion of a finger to the clearance between the central panel section and the finger picking section as well as picking of a finger to finger picking section. In either case, however, there is no difference in force for pulling up the tab, so that can openability is not improved to such an extent that children and the aged may open cans easily.
The present invention has been made to solve the above mentioned problems. Its object is to provide has an easy opening can end which is made of steel sheet coated with resin layers that are not damaged at the time of forming a score theron, and which has can openability in the degree that children and the aged may easily open the can end. In addition, the present invention provides a method for fabricating such an easy opening can end.
The object of the present invention may be achieved by an easy opening can end whose can end panel is made of steel sheet having both sides coated with resin layers which each comprise at least one type of thermoplastic resin and which have elongation after break of 100% or more, tensile strength of 10 kg/mm2 or more, Young modulus of 100 kg/mm2 or more and a thickness in range of 10 to 100 xcexcm. A score is formed on at least one of a front or a back side of the can end panel so as to be capable of breakage for opening the can end. The score has a bottom cross-section in a shape of an arc having a radius in a range of 0.10 to 1.0 mm, and a thinnest section of the score has a thickness t in a range of 0.025 to 0.080 mm.
Additionally, the easy opening can end described above according to the present invention may be fabricated by a step of forming a can end panel with steel sheet having both sides coated with resin layers which each comprise at least one type of thermoplastic resin and which have elongation after break of 100% or more, tensile strength of 10 kg/mm2 or more, Young modulus of 100 kg/mm2 or more and a thickness in range of 10 to 100 xcexcm, and a step of forming a score on the can end panel by press forming using a pair of dies, at least one of which has a shape of a curved surface with a radius in a range of 0.10 to 1.0 mm, and so that a thickness of a thinnest section t of the score is 0.025 to 0.080 mm.